I'd look toward restricting the oil coming from number 6 intake, it sounds essentially like this is a leak that is starving the rockers following it in oil feed order (back of engine to front). This is probably an issue with the lifter as it dictates the amount of oil entering the pushrod. In total there really isn't a lot of oil moving down the lifter galley so a lifter that isn't doing a good job metering oil to the rocker will starve everybody else on that circuit especially those after it.

We've seen a lot of lifter problems in recent years, having new ones is no assurance of a good part.

Bogie

Now that's just silly. It's more likely that that one has the least lifter-to bore clearance.

I'd say there's enough wear in the bores and the lifters are probably at the small limit size-wise. If there is enough oil flow at 1200 rpms, I'd not worry about it.